Soy & Your Thyroid: How Much Is Too Much?

Eating large amounts of soy foods may not be good for some women's thyroid functioning, according to new research from Loma Linda University in California.

However, another thyroid expert not involved in the study says the caution about not overdoing soy protein probably applies only to a minority of women and it should not cause concern for most.

In the new study, researchers tracked the diets of more than 800 men and women and then looked at their TSH (thyroid stimulating hormone) levels. Abnormally high TSH levels reflect a failing thyroid.

"The chances of having a high TSH were quadrupled in those who ate, on average, just under two servings a day [of soy foods] compared to those who didn't eat any," says study leader Serena Tonstad, MD, PhD, professor of public health at Loma Linda and a preventive cardiology physician in Norway.

She found no link between the soy intake of men and their thyroid health. And she stresses that she found only a link in women, but cannot prove cause and effect. "Our study is observational," she says. And women do have a higher risk of developing thyroid problems than men, especially as they age.

It may be that those with normal thyroid functioning will do OK eating soy foods, Dr. Tonstad says, "but in those who are borderline, it may take them above the threshold to give them hypothyroidism." The thyroid gland is crucial to regulate metabolism and other bodily functions.

The study is published in the June 2016 issue of Public Health Nutrition. The Soy Nutrition Institute partially funded the work but had no role in the report. One coauthor, Mark Messina, is executive director of the Institute.

Soy Study Details

The debate about soy and thyroid health has been ongoing. In the new study, researchers asked the 800 men and women, age 30 and above, and not on thyroid medicines, to keep track of their soy intake over a six-month period. The participants finished six 24-hour dietary recalls within the study period and also gave blood samples. The researchers evaluated soy intake from both foods and supplements.

The men and women in the study ate a variety of diets, but those with high TSH levels tended to be older, Dr. Tonstad found. Women with higher TSH levels were more likely to be vegans (no meat, poultry, fish, eggs, milk or dairy) or lacto-ovo vegetarians (dairy products and/or eggs but no fish or meat).

The women in the group eating the most soy averaged about 11 grams a day, or about two servings, compared to an average of no soy in the lowest group. The average U.S. consumption of soy is about 2 grams a day, Dr. Tonstad says. Two servings of tofu, each about three ounces, would provide 12 or more grams of soy protein.

Of the 43 women with high TSH levels, she found, 13 had reported a diagnosis of hypothyroidism within three years of starting the study, but weren't on thyroid pills now.

Overall, the men and women's average TSH levels was 2.6 mIU/l (under 5 is ''normal"). Those with high TSH levels averaged nearly 8.

Some experts have speculated that soy's isoflavones, which act like weak estrogens, may hamper the body's synthesis of thyroid hormones.

Soy: Second Opinion

The study findings are at odds with much other research, says Stephanie Lee, MD, PhD, director of the Thyroid Health Center at Boston Medical Center and associate professor of medicine, Boston University School of Medicine. She reviewed the findings but did not participate in the study.

"Most other studies have not shown an association of soy protein intake and abnormal thyroid tests," she says. The study also has limitations, Dr. Lee says. For instance, there is no information on which women may have been taking soy for hot flashes during menopause. "The older you are, the higher your TSH tends to be, and the more likely you have hot flashes," she says. So those eating the most soy may have had high TSH levels to begin with, she says.

Her bottom line? Most who are eating a normal healthy diet with soy and soy protein should not worry about their thyroid function. The only group who might take note of the finding, she says, are those with borderline low thyroid function as demonstrated by a TSH between 4 and 5, especially if they have a vegan diet low in iodine. The thyroid gland uses iodine from food to make thyroid hormones.

Soy: What to Do Next

Dr. Tonstad agrees that soy is a high quality protein, found in some research to help lower cholesterol and blood pressure. However, she says, women who have thyroid disease in the family and eat a lot of soy should get a TSH test, she says, to see where they stand.

As for men, she says, ''it doesn't seem like they need to do anything."

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